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This paper reviews the usual open boundary conditions (OBCs) for coastal ocean models and proposes a complete set of open boundaries based on stability criteria, on mass and energy conservation arguments, and on the ability to enforce external information. This set includes a radiation condition for barotropic variables, an equation of wave propagation for baroclinic velocities, and an advection equation for tracers. Considerations on mass and energy conservation properties suggest a suitable numerical treatment of the barotropic scheme, which is different from what is commonly used. Restoring terms, when classically added in the Sommerfeld OBCs, are not consistent with external fields. It is shown that this can be avoided if proper numerical schemes are used or if OBCs are applied on differences between the model and forcing rather than on absolute variables. Finally, this paper shows that simplistic advection-type methods for temperature and salinity should not be used in sigma coordinate models because this introduces errors in the computation of the horizontal pressure gradient. [ABSTRACT FROM AUTHOR]

We study the Gironde River plume in the Bay of Biscay (North-East Atlantic) using a high resolution modelling approach of the estuary-mouth-shelf continuum. The Gironde River plume flows over a gently sloping shelf where semi-diurnal tides reach an amplitude of 1.3–1.5 m on average; the Gironde River discharge undergoes a significant seasonal variability with floods (>1000 m3/s) in winter and spring. From two years of simulation (2011–12), we document the behavior of the plume under varying forcing conditions. We then discuss the dynamical mechanisms at play, from the estimate of some mechanical energy budget terms and using sensitivity runs: run with no tides, run with filtered atmospheric forcing (no variability at time scales smaller than 30 days) and run with no river runoff. We first show that the plume's properties are determined by the processes occurring in a transition area at the estuary outlet between the Royan and Cordouan sections. The outflow passes through a deep and narrow channel both because of bathymetric constraints and tides. Indeed, tides generate a residual circulation made of anticyclonic and cyclonic vortices with diameters of 5–12 km. Tides also induce intense mixing that strongly modifies the stratification between the two sections. The mean conditions lead to classify the plume as a narrow outflow which is a favorable condition for a bulge to be formed according to the literature. We indeed observe the recurrent formation of a bulge with sharp salinity gradients and a narrow coastal current. The bulge develops in moderate to high discharge conditions, during the spring to neap tides transition and when winds are weak. When the intensity of tides and wind increases, the bulge breaks down and the plume spreads offshore under upwelling wind conditions or along the coast (both in northward and southward directions) under strong eastward wind conditions. Tides are the main driver at the estuary outlet (with a bottom friction term larger by two orders of magnitude than over the adjacent shelf) but winds and atmospheric pressure are the main forcing over the inner shelf. These results evidence the need to model accurately the whole estuary-shelf continuum when studying the fate of riverine waters over the shelf. • An estuary-shelf continuum model is built to study the Gironde River plume dynamics. • Tides play a key role at the mouth due to residual circulation and strong mixing. • Bulge formed at spring to neap tides transition with low wind and significant runoff. • The plume extends offshore or north to south along coast depending on wind direction. • Tides and wind are the main drivers at the mouth and over the inner shelf respectively. [ABSTRACT FROM AUTHOR]

The design (location and size) of sustaining, no-take reserves was investigated by combining realistic numerical simulations of larval dispersal from a sedentary marine species with a population dynamics model. The method explored, a priori: (1) the planktonic larval duration (PLD) of self-persistent populations within no-take reserves with radii from 1 to 20 km, (2) the size of a no-take reserve reaching self-persistent recruitment of the reserve population, and (3) offspring spillover to adjacent fisheries for PLDs from 1 to 6 weeks. In the Gulf of Lions (northwest Mediterranean), as the radius of a no-take reserve increased to 20 km, the median PLD of a self-persistent species within the reserve increased from 2 to 6.5 d. No unique relation between PLD and sustaining no-take reserve size could be established because of large spatial and temporal variabilities, thus precluding any general guidelines for marine protected area sizes. For species with mass spawning lasting < 3 d, variability due to spawning timing yielded twice the spatial variability, reflecting strong wind variability. In contrast, when spawning lasted more than 10 d, the spawning location became more important. Thus, a biological process (spawning duration) can trigger deterministic and stochastic effects of environmental variability. Finally, some unprotected areas (Narbonne to Agde and the Camargue) clearly appeared to be better locations than the existing no-take reserves for maximizing biodiversity persistence within a reasonable no-take reserve size (10 to 20 km) and for producing offspring spillover important for regional fisheries (80%). [ABSTRACT FROM AUTHOR]

Abstract: The upper circulation in the southern Bay of Biscay is analysed over winter and fall 2004 using a coastal altimetric dataset, moorings, sea surface temperature (SST) observations and a regional simulation from the SYMPHONIE model. The aim is to determine whether altimetric data can detect occurrences of a slope current (the Iberian Poleward Current, IPC). We first analyse in situ and SST data. The results show pulses of a warm surface poleward flow (IPC) limited to the westernmost part of the northern Spanish coast. Along-track TOPEX/Poseidon and Jason-1 data are analysed over three ‘events’ (Jan., Feb., and Oct. 2004). Altimetric data can depict the main pulses. They provide some information on their spatial variability but the present in situ dataset is not dense enough to assess this information. Local inconsistencies between neighbouring tracks may be related to uncertainties in altimetric corrections. The simulated fields are consistent with the in situ observations. They allow us to better understand the IPC subsurface properties and its signature in altimetric data. The ability of altimetry to monitor IPC pulses under any atmospheric conditions or when the SST signature is weak makes it an essential component for the IPC observing system in addition to satellite SST and moorings data. [Copyright &y& Elsevier]

Abstract: The energy transfers associated with internal tide (IT) generation by a semi-diurnal surface tidal wave impinging on a supercritical meridionally uniform deep ocean ridge on the f-plane, and subsequent IT-propagation are analysed using the Boussinesq, free-surface, terrain-following ocean model Symphonie. The energy diagnostics are explicitly based on the numerical formulation of the governing equations, permitting a globally conservative, high-precision analysis of all physical and numerical/artificial energy transfers in a sub-domain with open lateral boundaries. The net primary energy balances are quantified using a moving average of length two tidal periods in a simplified control simulation using a single time-step, minimal diffusion, and a no-slip sea floor. This provides the basis for analysis of enhanced vertical and horizontal diffusion and a free-slip bottom boundary condition. After a four tidal period spin-up, the tidally averaged (net) primary energy balance in the generation region, extending ±20km from the ridge crest, shows that the surface tidal wave loses approximately C =720W/m or 0.3% of the mean surface tidal energy flux (2.506×105 W/m) in traversing the ridge. This corresponds mainly to the barotropic-to-baroclinic energy conversion due to stratified flow interaction with sloping topography. Combined with a normalised net advective flux of baroclinic potential energy of 0.9× C this causes a net local baroclinic potential energy gain of 0.72× C and a conversion into baroclinic kinetic energy through the baroclinic buoyancy term of 1.18× C. Tidally averaged, about 1.14× C is radiated into the abyssal ocean through the total baroclinic flux of internal pressure associated with the IT- and background density field. This total baroclinic pressure flux is therefore not only determined by the classic linear surface-to-internal tide conversion, but also by the net advection of baroclinic (background) potential energy, indicating the importance of local processes other than linear IT-motion. In the propagation region (PR), integrated over the areas between 20 and 40km from the ridge crest, the barotropic and baroclinic tide are decoupled. The net incoming total baroclinic pressure flux is balanced by local potential energy gain and outward baroclinic flux of potential energy associated with the total baroclinic density. The primary net energy balances are robust to changes in the vertical diffusion coefficient, whereas relatively weak horizontal diffusion significantly reduces the outward IT energy flux. Diapycnal mixing due to vertical diffusion causes an available potential energy loss of about 1% of the total domain-averaged potential energy gain, which matches to within 0.5%, for k m linearly distributed grid-levels and constant background density ρ 0, vertical diffusivity (K V ) and buoyancy frequency (N). [Copyright &y& Elsevier]

Abstract: Observations of dense water formation on the shelf of the Gulf of Thermaikos (North Aegean) are presented, based mainly on continuous monitoring of temperature and currents, during the winter of 2001–2002, at an instrumented mooring and a CTD survey carried out in early February 2002. A 2.5-month realistic simulation, corresponding to the period of observation, was performed to investigate the processes of dense water formation and cascading. The simulation is first compared to the main characteristics of the dense water, time variation of bottom temperature and spatial distribution of the dense water on the shelf. Subsequently, the simulation is used (a) to show that the formation of dense water takes place within the semi-enclosed Thessaloniki Bay and (b) to explain the intermittence of cascading out of the bay in relation to wind variations. The pathways of the dense water through the shelf are investigated with an emphasis on the role of the bottom slope and friction in the Ekman layer. The export of dense water towards the open sea occurs primarily along the slope bounding the western coast. [Copyright &y& Elsevier]

Abstract: A high horizontal resolution 3D hydrodynamic model was applied to a semi-enclosed bay (Gulf of Fos, western Mediterranean Sea) in order to describe the specific circulation patterns driven by winds and to infer the scales of residence times. Freshwater inputs to this bay come from the Rhône river and navigation channels. Idealised simulations under typical wind forcing conditions were performed and are described in this paper. They revealed several features of the exchanges of water masses in the Gulf of Fos, in particular the intrusion of the Rhône river plume generated by south winds. During northern wind conditions, surface waters flushed out of the system. To compensate for this outflow, bottom currents transporting marine waters took the opposite direction. Residence times in the different areas of the gulf were also investigated. A realistic simulation was performed under actual conditions of Rhône river discharges, meteorological forcing and impact of the surrounding general circulation. Model outputs, mainly salinity fields, were compared to 10-day observations acquired during a multidisciplinary cruise that took place in May 2001. Model results and observations were in good agreement and showed a massive inflow of the Rhône river plume waters into the Gulf of Fos during a south wind event. Sensitivity studies showed that the salinity field was strongly dependent on the wind direction and on the local freshwater discharges. [Copyright &y& Elsevier]

Secondary flows induced by the blocking effect of a river plume on a transverse upwelling are investigated in a microtidal region of freshwater influence (ROFI). A nested version of the SYMPHONIE primitive-equation free-surface model for 3-D baroclinic coastal flows has been developed for the Rhône ROFI. The main characteristics of the model are a generalized sigma coordinate system in finite differences, using a time splitting for external and internal modes and high-order numerical advection schemes for density fields in combination with an modified turbulence closure scheme. The nesting system consists of two grids forced by the high-resolution ALADIN model atmospheric data. The coarse grid of 3 km resolution for the whole Gulf of Lions allows the forcing of the Liguro-Provençal large-scale current when the fine mesh of 1-km resolution is centred on the river mouth of the Grand Rhône. Documented field experiments from the Biodypar 3 field campaign performed during March 1999 are used for validation. Numerical results, CTD profiles and a SPOT TSM visible image are in good agreement concerning the shape and structure of the river plume. Other coastal flow features can be observed from satellite imagery. Computations of realistic situations recover these main secondary structures. Complementary process-oriented runs give an explanation of how the coastal upwelling induced by an inhomogeneous offshore wind is destabilized by the combination of the river plume and along-shelf current-blocking effects. In the end, a factor-separation analysis provides evidence that the locally non-linear effects in momentum contribute to the occurrence of secondary vortices. [ABSTRACT FROM AUTHOR]

Free surface coastal models currently suffer from the difficulty of having to specify the global circulation during the initialization process and along the open boundaries. As an alternative to the long spinup periods, an original explicit approach based on inverse techniques has been developed. Data originating from in situ observations and/or ocean general circulation models are optimally interpolated over the small-scale grid in such a way that the tendency terms are reduced to physically consistent values. The errors on the "true" tendencies and the truncated nonlinear term are evaluated to compute the model covariance. The observation covariance matrix is divided into two parts: the homogeneous, isotropic matrix calculated with a global energy spectrum; and a parameterized nonhomogeneous, nonisotropic matrix. The inverse method is applied to the study of the interaction of a barotropic alongshore current over a narrow canyon. The transient processes following the initialization are drastically reduced and the analysis field can efficiently be used in a flow relaxation scheme along the open boundaries. [ABSTRACT FROM AUTHOR]

Downscaling physical processes from a large scale to a regional scale 3D model is a recurrent issue in coastal processes studies. The choice of boundary conditions will often greatly influence the solution within the 3D circulation model. In some regions, tides play a key role in coastal dynamics and must be accurately represented. The Bay of Biscay is one of these regions, with highly energetic tides influencing coastal circulation and river plume dynamics. In this study, three strategies are tested to force with barotropic tides a 3D circulation model with a variable horizontal resolution. The tidal forcings, as well as the tidal elevations and currents resulting from the 3D simulations, are compared to tidal harmonics extracted from satellite altimetry and tidal gauges, and tidal currents harmonics obtained from ADCP data. The results show a strong improvement of the M2 solution within the 3D model with a ”tailored” tidal forcing generated on the same grid and bathymetry as the 3D configuration, compared to a global tidal atlas forcing. Tidal harmonics obtained from satellite altimetry data are particularly valuable to assess the performance of each simulation. Comparisons between sea surface height time series, a sea surface salinity database, and daily averaged 2D currents also show a better agreement with this tailored forcing. [ABSTRACT FROM AUTHOR]

We show that a time-varying bathymetry can be implemented in a σ-coordinate free-surface ocean model with only slight modifications to the original numerical algorithm. A consistent choice of variables for the height of the water column is proposed. The resulting algorithm creates new opportunities in ocean model configurations in which bathymetry changes at time scales comparable with those of ocean dynamics (e.g., near shore bed motion and seismic or volcanic catastrophes) or in direct modeling of laboratory experiments with a moving bottom. The new numerical implementation is carefully compared with a laboratory experiment involving internal wave generation by an oscillating, ridge-shaped topography. An energy formulation of the time-varying bathymetry is proposed, and energy fluxes are computed to characterize and quantify the cascade of energy leading to the generation of internal waves by an oscillating ridge. [ABSTRACT FROM AUTHOR]

A Lagrangian module has been developed and coupled with the 3D circulation model Symphonie to study the influence of hydrodynamic processes on zooplankton transport and distributions in the North Western Mediterranean (NWM). Individuals are released every 3 days from March to August 2001 in two initial areas: around the DYFAMED sampling station in the central Ligurian Sea and in the Rhône river plume. Then the individuals are tracked for 40 days either as passive particles or with a simple diel vertical migration (DVM) pattern. The simulations suggest strong seasonal patterns in the distributions of the individuals released around the DYFAMED sampling station. Individuals spread all over the NWM basin after 40 days but different patterns occur depending on the season, the initial depths of release and the capacity of DVM. Offshore-shelf transport only occurs in April and May with particles ending up in the Gulf of Lions (GoL) in low concentrations. In other months, the Northern Current (NC) can be considered as a barrier for particles entering the GoL from the offshore sea. A quarter to a half of passive individuals released in the Rhône river plume remain in the GoL. The rest is transported by the NC towards the Catalan Sea. Applying a simple DVM scheme does not increase the retention of particles on the shelf. [Copyright &y& Elsevier]

Abstract: We present realistic simulations of mesoscale anticyclonic eddies, present in the western side of the Gulf of Lion and generally observed in satellite imagery during July and August. A nested model of 1-km resolution covering the Gulf of Lion is implemented from a coarse model of 3-km resolution. The models use an upwind-type advection–diffusion scheme, in which the numerical diffusion term is adjusted by an attenuation coefficient. Sensitivity tests have been carried out, varying the model spatial resolution and the attenuation coefficient to reproduce the (sub)mesoscale structures. A wavelet technique is applied to analyze the modelled horizontal relative vorticity in order to define the area, position and tracking duration of the eddy structures. Comparisons between the modelled eddies and those observed by satellite have allowed us to choose the best model setup. With this setup, the studied anticyclonic eddy lasted for 60days. [Copyright &y& Elsevier]

Abstract: The generation of internal tides in the ocean is due to the interaction of strong barotropic tidal currents with variable topography in stratified waters, transferring energy from the external to the deep ocean. The internal tides feed later the ocean mixing, playing a major role for the maintenance of the stratification of the global ocean. A remarkable region in terms of tidal energy is the European continental shelf. As a first step toward the study of internal tides in the Bay of Biscay, this paper aims at understanding the barotropic tides and associated energy budgets. On continental shelves and in coastal seas the use of regional models with fine grid resolution is preferred to the use of global tidal atlases derived from altimetry. The unstructured grid T-UGOm model is used to compute the NEA-2004 tidal solutions in the North-East Atlantic ocean, with errors greatly reduced in coastal areas compared with global models. Energy budgets are discussed based on the inclusion of nonlinearities in the tidal solutions. The sea surface height and depth-averaged currents are used to compute the tidal energy conversion from barotropic to baroclinic tides, tidal dissipation and energy flux. A total amount of energy of 250GW is found for the tide. The path of energy from the Southern Atlantic ocean toward the Bay of Biscay is highlighted, advocating for nonzero boundary conditions in regional models. The 3D coastal ocean SYMPHONIE model has been implemented to simulate the surface tides in the Bay of Biscay. Solutions are validated by comparison with the NEA-2004 solutions and observations. [Copyright &y& Elsevier]